Blender CAD commands
Specifically for the commands you mention there exists a CAD add-on called mesh tinyCAD, allows you to easily do the following things (this is not an exhaustive list):


VTX (like _extend):  


automatically extend edges towards their projected intersection
generate a vertex at the intersection of two edges  ("weld")

V2X:


generate an unattached vertex at the projected intersection of two edges

XALL:


intersects all selected edges.

BIX:


generates the bisector of two selected edges

CCEN:


given any three selected vertices (in one object), this can recreate a circular set of vertices of any number and scale.



For questions about the add-on or ideas use the project's issue tracker



For _fillet you can use the bevel command: CtrlShiftB With 1 vertex selected  


It starts out as a bevel but you can increase the number of segments to turns it into a fillet.
Adjust radius